Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
A survey of "direct thermal" imaging methods is given e.g. in the book "Imaging Systems" by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press--London and New York (1976), Chapter VII under the heading "7.1 Thermography". Direct thermal thermography is concerned with materials which are substantially not photosensitive, but are sensitive to heat or thermosensitive. Imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material.
Most of the "direct" thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
According to U.S. Pat. No. 3,080,254 a typical heat-sensitive (thermographic) copy paper includes in the heat-sensitive layer a thermoplastic binder, e.g ethyl cellulose, a water-insoluble silver salt, e.g. silver stearate and an appropriate organic reducing agent, of which 4-methoxy-1-hydroxy-dihydronaphthalene is a representative. Localized heating of the sheet in the thermographic reproduction process, or for test purposes by momentary contact with a metal test bar heated to a suitable conversion temperature in the range of about 90.degree.-150.degree. C., causes a visible change to occur in the heat-sensitive layer. The initially white or lightly coloured layer is darkened to a brownish appearance at the heated area. In order to obtain a more neutral colour tone a heterocyclic organic toning agent such as phthalazinone is added to the composition of the heat-sensitive layer. Thermo-sensitive copying paper is used in "front-printing" or "back-printing" using infra-red radiation absorbed and transformed into heat in contacting infra-red light absorbing image areas of an original as illustrated in FIGS. 1 and 2 of U.S. Pat. No. 3,074,809.
In direct thermal imaging the image quality is strongly dependent upon the tone response of the direct thermal recording material to the heating pulses image-wise applied to the heat source and in particular the dependence of image density upon the power input to the heat source. Fine tuning of the response of the material enables image quality to be obtained whether continuous tone images are desired, for which a fairly large number of grey levels are required and therefore a moderately flat response is necessary, or graphics imaging is desired, requiring a single image tone and a very strong dependence of image density upon the power input to the heat source.
In EP-A 687 572 the incorporation of certain ingredients is disclosed, which enable the tone response (=image density) of a direct thermal recording material to the power input to the heat source to be made flatter thereby enabling a fairly large of grey levels to be attained, as required for continuous tone images. However, a means of steepening the response of direct thermal recording materials is equally desirable, but has not yet been found.